The corepressors NCoR and SMRT have been documented to have opposite effects on the EC50 of agonists, and the amount of residual agonist activity of antagonists, for GR and PR induction of the same gene in the same cells. These inverted responses depended upon the joint actions of the N- and C-terminal domains of each receptor (Song et al., 2001, J. Biol. Chem., 276, 24806-24816). These results are consistent with the demonstration that corepressors interact with N-terminal regions of both GRs and PRs ( Wang et al., 2007, Biochemistry, 48, 8036-8049; Wang and Simons Jr., 2005, Mol. Endo., 19, 1483-1500) in addition to the initially defined sites in the C-terminal sequences of receptors.[unreadable] [unreadable] The objective of this study is to examine the consequences of several other factors known to alter the EC50 and partial agonist activity of transiently transfected GR complexes and see if, under the same conditions, divergent responses are observed with the full-length PR, PR-B. The factors that we selected are: GME, GMEB2, Ubc9, and STAMP. GME, the first modulatory factor identified (Oshima and Simons Jr., 1992, Mol. Endocrinol., 6, 416-428), is a cis-acting DNA sequence that was isolated from the rat tyrosine aminotransferase (TAT) gene and acts in concert with two proteins that bind to the GME, i.e., GMEB-1 and -2 ( Oshima et al., 1995, J. Biol. Chem., 270, 21893-21910; Zeng et al., 1998, J. Biol. Chem., 273, 17756-17762; Chen et al., 2002, J. Biol. Chem., 277, 22053-22062). Ubc9 is the human homolog of the E2 ubiquitin-conjugating enzymes of yeast that can transfer a ubiquitin-like molecule, called small ubiquitin-like modifier-1 (SUMO-1), to proteins in vertebrate cells. However, Ubc9 also displays non-enzymatic effects on a variety of cellular actions including GR-mediated transcription (Kaul et al., 2002, J. Biol. Chem., 277, 12541-12549; Cho et al., 2005, Mol. Endo., 19, 290-311). STAMP is a novel protein that was cloned by its ability to augment the modulatory activity of the coactivator TIF2 in both GR-mediated induction and GR-regulated repression of target genes (He and Simons Jr., 2007, Mol. Cell. Biol., 27, 1467-1485). [unreadable] [unreadable] The current studies demonstrate that the GME and GMEB-2 differentially alter several of the above induction parameters of GRs and PRs under otherwise identical conditions. Thus the GME modifies induction properties of the transiently transfected reporter GMEGREtkLUC by GRs but not by PRs in 1470.2 mouse mammary adenocarcinoma cells. Similar selective effects were displayed in the presence of transiently transfected GMEB-2. Increasing amounts of GMEB-2 provoked a concentration-dependent increase in the EC50, and decrease in amount of residual agonist activity, or GR complexes. In contrast, opposite effects on PR induction properties were seen at low vs. high GMEB-2 concentrations. Initial results indicate that PRs are more sensitive to the modulatory activity of Ubc9 than are GRs. Thus, the number of factors that unequally modulate PR and GR induction parameters is now expanded, thereby increasing the possible mechanisms for differential gene regulation by PRs vs. GRs. These results also support the hypothesis that the modulation of EC50, partial agonist activity, and Vmax by a given factor is not limited to one receptor in a specific cell line and is a more general phenomenon. [unreadable] [unreadable] In summary, progress has been made in understanding the different biological properties of PRs and GRs, even when they bind to the same HRE. We have reported that the coactivator TIF2 and the corepressors NCoR and SMRT competitively bind to different N-terminal regions of PR and GR in a manner that contributes to their different biological responses to corepressors (Wang et al., 2007, Biochemistry, 48, 8036-8049). The number of cofactors that can differentially perturb the EC50, partial agonist activity, and Vmax of PR vs. GR is expanded by the current study. These modulatory factors permit a continuum of receptor-selective responses and constitute new therapeutic targets for differential control of gene expression by steroid hormones during development, differentiation, homeostasis, and endocrine therapies. These combined findings contribute to our long-term goal of defining the action of steroid hormones at a molecular level and of understanding their role in human physiology.